2012 6th Int Symp on RCC Dams-Innovations of Significance and Their Development on Some Recent RCC...

8/13/2019 2012 6th Int Symp on RCC Dams-Innovations of Significance and Their Development on Some Recent RCC Dams-ForBES (1)

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6THINTERNATIONAL SYMPOSIUM ONROLLER COMPACTED CONCRETE (RCC) DAMS

Zaragoza, 23 25 October 2012

Invited Lecture

INNOVATIONS OF SIGNIFICANCE AND THEIR DEVELOPMENT ON SOME

RECENT RCC DAMS

Brian A. FORBESManager, Major Dams Projects, GHD Pty Ltd, Australia; [email protected]

SUMMARY

This paper deals with what the author believes are some of the more

significant innovations that have been adopted on RCC dams in recent timeswhich provide for improved quality, more efficient construction and reduced costs.

These include the sloped layer construction procedure and use of grout enriched

RCC in place of conventional vibrated concrete and some recent new approaches

to their use. It also includes some other interesting procedures used recently,

such as slip forming GERCC for the top step on stepped downstream faces and

spillways, the construction of galleries by excavation using a rock trenching

machine, and the use of precast concrete elements for the spillway ogee crest

structure.

1. INTRODUCTION

Over 500 large RCC dams have been developed in the world[1]since the

advent of RCC to the dam design and construction industry with the completion of

Willow Creek dam in the USA in 1982. As with any dam, each RCC dam is unique

in its setting and consequent design. It is with respect to this uniqueness that RCC

technology has developed through innovation to its current state of the art.

In the evolution of RCC design and its construction process, a huge number

of different approaches have been trialed, with varying degrees of success,

involving both the very basics of RCC including design concepts, methods of

transporting and placing RCC, construction of upstream and downstream faces

etc, as well as many of the minor details.

It is this through this process of innovative trialing and regular reporting at

international symposia, such as this one, that we are now reaching some

consensus, particularly on the basic aspects, as is evident in the Chinese

approach to RCC design and construction. However it is important that there

always remains the opportunity for innovative thinking and trialing, as this is what

drives us as engineers, makes the subject of RCC as stimulating as it is and will


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continue to deliver improvements in quality and reductions in construction time and

project costs.

The principle concern that RCC dam owners, designers and regulators have

with regard to RCC relates primarily to the high number of lift joints and theirpotential for leakage and reduced strength when compared to CVC construction.

Generally an RCC dam placed with 300mm thick lifts will have 5-8 times the

number of lifts than an equivalent CVC dam. The need to achieve monolithic

construction across lift joints is particularly critical with higher dams and those in

high seismic regions. The method of placing RCC lifts using sloping layers is an

innovative and effective solution which also delivers many other benefits to the

RCC construction procedure. A very simple in-situ transformation of RCC by the

addition of a cement grout will achieve an impermeable, durable outer facing of

GERCC to the upstream and downstream faces of the dam and spillway, as well as

for use in other parts of the dam, thereby eliminating the need for a separate CVC

mixing and delivery system. Both of these innovative developments are discussed

in detail in the paper as well as some recent new approaches to their use.

Selection of other innovations for inclusion in this paper has been on the

basis of some recent Australian RCC projects which have been developed under

Alliance Contracts[2], which tend to drive innovative thinking and ideas and their

adoption for the benefit of the project and the Alliance partners (Owner, Engineer,

Contractor). These include the use of a slip form paver to finish the GERCC on the

wide top of downstream face steps to reduce hand labour and improve surface

tolerances; excavating horizontal galleries though hardened RCC using rock

trenchers, and; pre-casting concrete spillway ogee crest elements to acceleratethe completion of the crest works and reservoir impoundment.

1. THE SLOPED LAYER METHOD

A significant difference between a CVC dam and a RCC dam is the number

of horizontal construction lift joints. Formed conventional concrete is generally

placed in 1.5-2.5m lifts, whereas RCC is generally placed in 0.3m lifts, i.e. with

RCC there are 5-8 times more lift joints and potential planes of weakness alongwhich seepage, sliding or overturning failures could occur.

Various approaches have been adopted in earlier RCC dams to achieve

bond between lift joints, including minimizing RCC segregation and damage to the

compacted lift surface by construction plant, maintaining curing, pre-preparation of

the surface, final surface clean up and use of a bonding concrete, mortar bedding,

or grout applied over all or part of the lift. Another approach is the use of RCC

mixes containing a high fly ash or pozzolan content, relying on the delayed

hydration and cementation process of the fly ash or pozzolan to assist in the

bonding process with the overlying lift, or/and use or set retarding admixtures.


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The maturity index, which is a function of lift surface age and ambient

temperature to which it has been exposed, has been used to define the difference

between hot, warm and cold joints and the degree of treatment required to the

lift joint surface. However since it is unusual to be able to place more than one or

two RCC lifts per day over the full area of the dam, the initial and final set of thecement will have well and truly occurred before the next lift is placed, especially

with un-retarded RCC mixes. As a consequence, the ability of the surface of the

lower lift to develop full bond with the under surface of the overlying lift, is largely

lost.

Vertical cores taken through RCC lift joints commonly show one in every two

joints recovered broken, i.e. the bond was insufficient to overcome the torque

applied by the coring bit. Direct tensile strengths of bonded joints have been shown

to vary considerably, seldom are joints more than 1/3rdof the parent RCC tensile

strength, those using a bedding mortar may reach 2/3rds, but it is very unlikely that

aggregate particles from the overlying lift will penetrate the stiffened surface of a

lower lift. Cores across lifts where waterproof paper was placed confirm this.

With this background experience it is now generally accepted that unless the

overlying lift can be placed within the initial set time of the RCC then lift joint

strengths must dominate the design and will determine the potential failure plane.

Hence the parent RCC strength itself is, in fact, of lesser importance.

Two basic approaches have evolved. One is to heavily retard the hydration

process by use of a commercial chemical admixture, the other being to significantly

reduce the surface area of the placed lift by constructing the dam in blocks or byusing the method of sloping layers.

Chemical retarders were first used in RCC in China in the mid 1990s; an

inexpensive product of the sugar industry developed for the purpose. Retardation

extended initial set from 1.5-2.0 hours (unretarded) to 6-8 hours depending on

summer/winter temperatures. The placing process involved forming up for a 3m

thick lift with the form aligned on a transverse contraction joint, such that the area

enclosed resulted in a 300mm thick RCC layer with a volume equal to or less than

the volume of RCC that could be produced and delivered in the retarded initial set

time, as was adopted initially for Jiangya Dam in 1997 [3], Fig 1.

In this way 10 layers of RCC would be placed all within the retarded initial set

time of the RCC. A gap in the form and a sloped RCC ramp allowed access into the

placement area. On completion of the lift the transverse form would be re-erected

and the adjoining 3m lift placed. The surface of the cold lift joint would be greencut

and a bedding mortar applied with the first RCC layer. The process was

encumbered with the cost and time of setting up the transverse form and the cost

of the retarding admixture, but this was offset to some degree by the time and costs

saved in lift joint preparation.


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Fig.1Jiangya Dam, 131m high, 1.1 mill m3China 1997, placing 10 layers of RCC in a

formed up 3.0m block.

The block process was developed on more recent projects to one of placing

fewer layers, generally four, to form a 1.2m thick lift. By using pre-cast concrete

blocks as the tranverse form, time and costs were reduced, but it involved more

cold joints for preparation, e.g. at Koudiat Acerdoune Dam in Algeria, Fig 2.

Fig. 2

Koudiat Acerdoune Dam, 121m high, 1.65mill m3Algeria 2007, placing four RCC

layers with pre-cast concrete side forms and steel access ramp for a 1.2m lift.


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The main advantage of this method is that it simplifies the forming up of the

downstream stepped face of the dam, which is complex when lifts heights exceed

the step height adopted for the steps.

On some projects that are set up for very high RCC placement rates, oftenusing a high pozzolan or fly ash content in the RCC mix which, together with a

retarding admixture, can achieve delayed initial set times of 18-24 hours, it is

possible to place a single 300mm lift over the full area of the dam within the

delayed initial set time of the previously placed lift and there is no need to divide the

dam into blocks. In which case, the placing rate, selection of plant and retarder

dose rate is based on the maximum lift volume of the dam which typically occurs

at about 1/3rdheight for dams with uniformly sloping abutments. For this method

placing rates for large dams of up to 8-10,000m3/day are usually necessary, e.g. at

Yeywa Dam in Myanmar.

The sloped layer method (SLM) of placing RCC was conceived during the

construction of Jiangya Dam in late 1997 [3] and adopted from about mid height

onwards. Placing rates increased considerably and the project was thus competed

on target as a direct result of changing to the sloped layer method. Since then the

method has been used with similar success on many other RCC dams in China

and others internationally.

The procedure initially adopted on Jiangya had been to place the RCC in 3m

high blocks using a transverse form as described above. By removing the

transverse form and placing the 300mm thick layers of RCC on a slope, in a

direction parallel to the axis of the dam, from one abutment to the other betweenthe formed upstream and downstream faces, as shown in Fig. 3 below, the same

3m lift could be built up as a continuous process across the entire dam without the

need for the transverse form.

Fig. 3

Explanation of the Sloped Layer Method.


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Simply changing the slope selected for the layers alters the volume of RCC

placed in the 300mm thick sloping layer. For example, using a 3m high SLM, as

adopted at Jiangya, with an RCC mixer output of 500m3/hr, an initial set time of the

un-retarded RCC of 2 hours and a width between upstream and downstream faces

of W, then the slope S of the layer, as shown in the Figure 3 below, can be foundfrom:

S < 2x500 i.e. S < 1000 approxWx3x0.3 W

Hence, for the example above, at lower elevations, where the width W is say

100m, then a slope of 1 on 10 is required. Later, when the width has reduced to say

25m at upper elevations nearer the crest, then a flatter slope of 1 on 40 could be

adopted, if the time between placing RCC layers of only 0.5 hour with a slope of 1on 10 is considered too short. Trials have shown that slopes should not be steeper

than 1 on 10 as the vibrating roller will tend to shear the RCC surface.

When using SLM the final clean up and preparation of the lower lift surface,

including application of bedding mortar, is restricted to a narrow strip along the toe

of the sloped layer where it contacts the previous lift surface. For slopes of 1 on 10

the width of the strip is about 3m and for slopes of 1 on 40 it is about 13m. The

newly completed horizontal lift surface behind can be green-cut whilst the RCC is

still young and the upstream (and downstream) face forms can be lifted, between

5-10 days would be available to prepare for the start of the next 3m lift. Lift surface

preparation and form lifting are thus effectively removed from the critical path.

If the 300mm thick sloping layers are placed within the initial set time of the

RCC no surface preparation, clean up or bedding mortar is required prior to placing

the next sloped layer. For 3m high lifts this reduces the surface preparation

required by 90%. It also reduces the number of lift joints (and potential failure

surfaces through the RCC dam) by 90%. Using sloped layers to build up 3m high

lifts therefore results in half the number of lifts which would occur in a conventional

concrete dam constructed using 1.5m high concrete pours.

To overcome the existence of a series of feather edges at the toe of eachsloping layer, as the layers run out onto the lower 3m lift surface, the solution

derived at Jiangya Dam, Fig 4, was to first place and compact a 4-5m wide

horizontal layer 150-300mm thick on the top of the previous lift as a strip or foot

along the toe of the sloping layer over the prepared lift surface.The sloping portion

of the layer then commences from about the centre of the foot. If necessary the

front of the foot can later be trimmed back by 100-200mm to firm RCC as part of

the surface preparation work that is progressing ahead of the advancing sloped

layer construction, being covered with bedding mortar just prior to placing the

adjoining foot for the start of the next sloping layer.


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Fig. 4

Jiangya Dam showing the SLM using 3m high lifts.

Similar feather edges will also occur at the top of the sloping layers, as they

run out at the top of the 3m high lift, these may also need to be cut back to

50-70mm thick as part of the lift joint preparation process. This is easily achieved

using high pressure air-water jetting to break away any poorly bonded featheredge material during the preparation of the hardened lift surface.

Besides ensuring improved lift joint quality, the sloped layer method removes

most of the ancillary items of work from the critical path; surface preparation, curing

and lifting of formwork can all be carried out independent of the RCC placing. In

addition, the amount of time available for lift surface clean up and preparation will

be increased up to ten fold when 3m high lifts are adopted.

The slope of the layers is controlled during placing by lines painted on the

upstream and downstream forms and by survey methods. Trucks have been used

on most of the projects but the all conveyor system using a crawler placer hasbeen successfully used on the Koudiat Acerdoune Dam in Algeria when the sloped

layer method was adopted for the 1.2m high lifts, Fig.5.

Since the 300mm thick layers of RCC are placed within the initial set time of

the previously placed layer, a greater RCC loading on the formwork will be

experienced before the lower layers reach final set condition. The design of the

upstream and downstream formwork and its anchorage back into the RCC needs

to take account of this increased loading when using SLM.


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Fig. 5

Koudiat Acerdoune Dam, using a crawler placer for SLM in 1.2m high lifts.

For inclined downstream faces, one of the approaches until recently was to

use vertical steps. If formwork is used then, for simplicity, the SLM lift height would

be made equal to the step height. At Jiangya Dam and also Kinta Dam (90m high

0.9mill m3, Malaysia 2006) where 3m high SLM lifts were used, precast concrete

blocks were used to form the 1m and 0.6m high steps on the downstream face

respectively[4]. Blocks were simply recovered from behind and added ahead of the

advancing layers as the horizontal RCC steps were constructed and a base for the

blocks to form the next step above became available. This stepped precast block

formwork system would appear to be an ideal method where more than one step

is required to match the selected SLM lift height, see Fig.6.

Fig. 6

Kinta Dam, pre-cast block forms for 0.6m d/s steps for SLM in 3m lifts.


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More recently at the Bui Dam (108m high, 1.0mill m3, Ghana, 2011) the

downstream face was formed using a full 3m high inclined form with SLM used to

place 3m high lifts. In the overhang, or shadow, zone where it was not possible to

get the large roller in to compact the RCC, the zone was constructed using grout

enriched RCC as described later in the paper, see Fig. 7.

Fig. 7

Bui Dam, inclined downstream formed face using 3m SLM lifts and GERCC under

the shadow areas inaccessible to the roller.

Placing a sloped layer generally involves commencing at the downstream

face and moving across to the upstream face (or visa versa), placing over the full

height of the lift and compacting in the up-down slope direction. The cross fall of

2-3% generally adopted in placing the traditional horizontal RCC layers for

drainage purposes during construction, can be retained when using SLM, i.e.

sloped layers will have a true slope directed slightly upstream (or downstream) of

the dam axis.

When commencing RCC placement and coming up off the foundations it is

appropriate to use the horizontal layer method. Once a width of about 20-25m

between abutments is reached the sloped layer method can be initiated with the

layers being sloped from downstream to upstream, i.e. parallel to the dam axis. For

the larger dams, the placement area can be divided into two or more blocks and

the sloping layers placed from downstream to upstream, as was done at Murum

Dam in Sarawak, Malaysia (141m high, 1.6mill m3

, Sarawak Malaysia 2011), seeFig. 8.

Later, when sufficient height of dam has been reached, such that the

distance between the upstream and downstream faces on the placement area

equals the distance between the abutments, i.e. the placement surface is

essentially square, then the direction of the slope of the layers should be changed

to slope from abutment to abutment, i.e. in a direction normal to the dam axis.

Placement would continue in this manner until such a height were reached that the

width of the placement area had reduced to 10-15m, i.e. near the crest of the dam,

at which point placing would revert back to horizontal layers.


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Fig. 8

Murum Dam using SLM with a 3m thick lift to place RCC in a block from

downstream to upstream in the river channel area, note the inclined, formed

upstream face adopted for the lower elevation of the dam (right).

Cores of RCC extracted from Jiangya, and more recent dams using the

sloped layer method, where a thin layer of bedding mortar has been applied on any

layer or lift joints older than the initial set time of the RCC, are at times being

recovered in single, un-broken lengths up to 15m long, see Fig. 9 and 10.

Fig. 9

RCC core up to 5m long from Jiangya Dam by SLM (left) and 15.1m long from

Longtan Dam (right) using horizontal lifts placed within the retarded initial set time.


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2. GROUT ENRICHED RCC

The difference in the proportions of the ingredient materials of conventional

internally vibratable concrete (CVC) and RCC is that CVC has a greater quantity ofcement and water than RCC. By simply adding additional cement and water to

unconsolidated, freshly spread RCC in the form of grout, such that it is reasonably

distributed throughout the RCC, the RCC can be mobilized and the grout uniformly

worked through the RCC during consolidation by a poker vibrator. This is the basic

concept of grout enriched RCC (GERCC).

For the GERCC process to work, the applied grout needs to fully drain down

into the spread RCC lift, to do this it is essential that the RCC is in a loose, as

spread condition. Usually it is necessary to trim back by hand the low windrow left

by the dozer blade that develops along the forms and to roughly level off the

surface of the RCC to that expected of the final GERCC surface before applyingthe grout.

Fig.11

Some examples of GERCC used for upstream facing, left Ralco Dam (155m high)

in Chile and right Kinta Dam (90m high) in Malaysia.

Grout penetration and distribution can be assisted if the RCC is hand rodded

using a length of 12 mm diameter reinforcing rod, say at regular 200-250mmintervals, to full depth of the lift.

During these activities, and at all stages up until the poker vibrator is inserted

into the RCC, it is essential that the RCC remain in its loose state and no

pre-compaction occurs, either by workers feet, or by the vibratory drum roller

getting any closer than about 1.5m to the GERCC zone. The adjoining zone of

RCC should only be compacted after the GERCC has been compacted.

Exposed final GERCC top surfaces can be finished to a smooth level surface

by first tamping with a long timber plank on edge, to level up the surface, after

which it can then be wood-floated to final surface. GERCC lift surfaces may need


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to have any residual grout/laitance removed, according to the specification

requirements for the lift surface of the RCC. If the next lift is to be placed within a

few hours the poker vibrator will re-penetrate the lower GERCC lift and the lift joint

will disappear, in which case there is no need to remove any laitance.

The quantity of grout required can be determined by laboratory or field trials.

About 8 litres/m/400mm facing width for a 300mm thick RCC lift has been found to

be adequate where the parent RCC has a VeBe of 15-20 seconds. At La Miel I

Dam (190m high 1.7mill m3, Colombia, 2002) the dryer 125kg/m3 cemetitious

content RCC mix, which had a VeBe time in excess of 40 seconds, required about

10-12 litres/m/400mm width. By making the water cement ratio of the grout equal

to that of the RCC, similar compressive strengths will be achieved for the GERCC.

Admixtures such as water reducers, set retarders and plasticisers can be added to

the grout if necessary. At both La Miel I and the Ralco Dam (155m high 1.6mill m3,

Chile 2003) a superplasticiser was used to enable thicker grouts to be used than

the usual 1:1 water cement ratio mix, which had previously been found just capable

of percolating down into the spread parent RCC. The increased cement content

grout gave a slightly higher strength to the GE-RCC. Marsh cone viscosity testing

was used to determine the quantity of superplasticiser required to obtain a

viscosity of about 34-36 seconds, similar to that of the 1:1 grout mix. The viscosity

of the grout must be such that it will flow into the voids of the unconsolidated RCC

lift, not pool on the surface.

During compaction the surface of the GE-REC will become mobile underfoot

with air bubbles coming to the surface - indicating that compaction is taking place

effectively, just as seen during the consolidation of CVC. If this is not evident thenmore grout is necessary and dose rates need to be adjusted. On removal of the

vibrator, any holes left by the poker should be tramped to close them up, or if this

is not able to be properly done grout dose rates should be further increased.

Fig.12

Application of grout and poker vibration of GERCC at Kinta Dam.


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The ultimate objective is too keep grout dose rates to a minimum if strength

and full compaction of the RCC/GERCC interface, is to be achieved. Slump cone

testing of freshly consolidated GERCC sampled from the facing should have a

slump between 5-20mm to avoid drying shrinkage cracking and to ensure proper

compaction along the GERCC - RCC interface. In some instances it may be foundthat in fact no grout is needed to enable consolidation by poker vibration, as

experienced at Cadiangullong Dam (43m high 0.12mill m3, Australia 1997) when

the parent RCC was fresh and highly workable with a low VeBe time (


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The size of poker vibrator is dependent on the maximum aggregate size,

workability of the original RCC, quantity of grout etc. At Jiangya dam a set of 4 x

150mm diameter poker units mounted on a transom attached to a mobile rig were

used. This was originally provided for large conventional concrete pours; it was

more than was necessary for the 300mm thick GERCC lifts. Elsewhere, such asCadiangullong and Kinta dams, poker vibrators as small as 50mm diameter were

used successfully. Two pokers side by side are often more effective than each

operating independently.

Following consolidation of a reasonable length of GERCC, about 10-15m,

roller compaction of the adjoining RCC, using the usual large vibrating rollers,

should take place right up to and overlap the first 50-75mm of the GERCC so that

the contact between the two is fully compacted. The GE-RCC will be displaced

upwards by the roller, but this is of no concern if it is less than 20-30mm, otherwise

the grout dose rates should be reduced, Fig. 13.

At the Cadiangullong Dam in Australia the lower 0.3m lift of the 0.6m high

spillway steps were done in GE-RCC and the top lift in CVC, as it was perceived by

the contractor to be more easily finished to the required tolerances. Later at the

Tannur (60m high 0.25mill m3, Jordan 2000)[4]and Kinta dams the full 1.2m and

0.6m high spillway steps (respectively) were constructed using GERCC, initially

using the usual horizontal RCC placing method with a later change made to the

sloped layer method, see Fig. 14.

Fig. 14GERCC spillway steps, left Kinta Dam 0.6m high after 6months of spillage, right

Tannur Dam 1.2m high, no spillage yet.

At Ralco dam the 0.6m high downstream face steps were constructed in

GERCC to provide a superior and more durable quality than plain RCC so as to

better resist the colder temperatures and potential freeze-thaw problems, Fig.15.

During construction the dam was overtopped on two occasions for nearly a week,

flows of up to 500m3/s, 1m deep were experienced with absolutely no damage to

the young GERCC facing.


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Fig. 15

Ralco Dam (155 m high) GERCC downstream face with 0.6 m high steps.

At Wadi Dayqah dam (75m high, 0.6mill m3. Oman 2009) the 1.2m high

unreinforced GERCC spillway steps were overtopped by 6m with no damage other

than the removal of the thin surfacing of mortar from the top of the steps and a few

small chips from the edge of the step, most likely due to debris impact. The top stepwas constructed out of GERCC-M (discussed later) to facilitate placing the wide

top of the step to achieve a smooth uniform finish, Fig 16. The average GERCC-M

cylinder strength was 25.7MPa at 270 days, 30% higher than the parent RCC.

Fig.16

Wadi Dayqah downstream stepped face and spillway of GERCC-M, spilling to a

depth of 6m shortly after completion.

At the Enlarged Cotter Dam (85m high, 0.4mill m3, Australia 2012) the dam

discharged up to 2m over a 9 day flood during construction when it had reached a

height of 40m. The top surface of the 1.2m high GERCC faced steps were


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The embedment of horizontal PVC waterstops in GERCC facing was very

successfully undertaken at Tannur Dam[5]along the connection of the RCC dam to

the CVC spillway apron, Fig.18.

Where reinforcing steel has been incorporated in GERCC, such as aroundgalleries as at Murum dam and in the upstream face as at Wadi Dayqah dam, the

steel appears to be just as well encased in the GERCC as in CVC. The feasibility of

incorporating reinforcing into GERCC in such locations, as well as in other areas,

has proven to be simple, straight forward and effective.

Cores taken horizontally through the facing and into the parent RCC behind,

consistently show fully compacted GERCC, often with no clear indication of the

transition to the RCC, which appears monolithic with the GERCC. Likewise

horizontal cores taken across lift joints have consistently shown excellent bond

between GERCC lifts, Fig. 19.

Fig.19

Horizontal 150mm diameter cores from Kinta Dam spillway through 400mm of

GERCC and into RCC, core on left is taken along a lift joint.

Quality assurance for GERCC comprises sampling of the GE-RCC after

compaction for slump testing and manufacturing of test samples for strength

testing. Grout density by density balance will confirm the cement content and if the

grout is not continually agitated then a test for grout stability can also be done by

simply observing the quantity of cement settling out of the grout mixture, between

time of mixing and using, from a sample stored in a clear plastic bottle. Horizontal

coring through the face, either within the body of the lift or along the lift joint itself,


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into the parent RCC behind will confirm homogeneity and density of the GERCC,

its lift joint strengths and the quality of the transition between GERCC and the

RCC.

On some recent projects, in order to significantly increase GERCC strengthsfor additional durability, or to be able to effectively add freeze-thaw admixtures to

GERCC, both of which are not able to be effectively incorporated into GERCC via

the added grout, the GERCC has been specially pre-mixed, either with all the

proportioned materials and additives mixed together at the CVC mixing plant, or by

feeding mixed RCC into a truck mixer on the dam and adding the additional

materials into the mixer drum and mixing them in with the RCC. The latter has the

advantage in that it overcomes special additional mixing and transportation plant.

This pre-mixed GERCC is being referred to as GERCC-M. Test results

show it is a superior material displaying a higher strength (without additional

cement) and as it can be superplasticised to achieve a higher slump, it is more

easily placed and the surface finished off for the wider top steps associated with

higher downstream steps. It has been used for the latter reason at the Wadi

Dayqah and Al Wehdah dams. At the Enlarged Cotter dam GERCC-M was used

an economical foundation leveling concrete, being superplasticised so it could be

pumped into position, but yet displayed set elastic properties similar to the parent

RCC and, like the GERCC, had minimal potential for drying shrinkage cracking.

Advantages of GE-RCC include:

Provides a durable, impervious, high quality off-form finish for upstream

and downstream facing to the body of the RCC dam,

Forms a homogeneous and monolithic mass with its adjacent parent RCC,

The entire procedure is simple, easily controlled and does not control the

progress of RCC placing,

A separate batching, mixing, transportation system is not required, unlike

with CVC,

Grout can be mixed by hand or by grout plant; tests on projects to date

confirm the uniformity of GERCC is similar to the parent RCC. Coefficientsof Variation below 10% have been achieved for compressive strengths,

Ability to incorporate reinforcing steel, waterstops, pipe encasements etc.,

Can be used between abutment rock and the RCC body to achieve good

bond and the filling of all rock cavities, irregularities etc.,

Pre-mixing in a truck or CVC mixer enables freeze-thaw and other

admixtures and extra cement etc to be added (i.e. GERCC-M) as required.

Low cost.


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Requirements and Limitations of GE-RCC include:

Quality control relies on inspection, an understanding of the requirements

by those applying the grout and doing the vibration, the inspection and

repair of any defective zones when evident on stripping of the forms andcontrolling grout dose rates,

Lift joint treatment is necessary, as with any conventional concrete lift

surface,

Achieving a toweled, level surface, say for exposed step surfaces, is not as

easily achieved as with conventional concrete since it is less workable,

using GERCC-M assists in this regard,

Where transverse joint waterstops are incorporated it may be necessary to

locally widen/transition the adjacent facing width to facilitate the large RCCdrum rollers negotiating around the waterstop installation.

3. SLIP FORM PAVING THE TOP OF GERCC STEPS

Using GERCC for constructing downstream stepped facings, including

unreinforced spillway steps, has been successfully adopted on many RCC dams,

as described above. However with higher steps heights (1.2m or more) it has been

found that the width of the top step becomes too wide (1.4m or more) to be readilycompacted, leveled and surface finished by hand. Also, a common problem found

with most horizontal downstream steps is that any seepage or light rainfall tends to

collect in the low spots along the top of the steps rather than drain off uniformly,

which then results in an unsightly aspect, including vegetation growth, to an

otherwise pleasing downstream face.

On the Enlarged Cotter Dam, presently under construction in Australia, the

1.2m high GERCC faced downstream steps were detailed to be finished with the

top step having a drainage cross fall of 1% and the outer corner finished with a

50mm 450chamfer. The contractor proposed the use of a small slip form paver

(Gomaco Commander III) that was available off a recent pedestrian pavement job.

The paver used a gang of 5 poker vibrators to compact the in place GERCC, after

the RCC had been pre-dosed with grout in the usual manner, and a surface screed

plate to finish the top surface according to the designed cross fall. The final surface

was then given a light hand trowel finish and dams transverse contraction joints

notched into the surface as required, Figs. 20 and 21.

The paver tracked along the adjacent leveled of and lightly pre-compacted

RCC placed just ahead or the advancing paver. The GERCC along the vertical

step form was pre-compacted with a hand held poker vibrator to ensure that a good

off form surface was always achieved.


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Fig. 20

Slipform paver, shown compacting GERCC for the top lift of the step using poker

vibrators; the paver is moving towards the camera.

Fig. 21

Finishing off the slipformed surface of the top lift and the final aspect of the stepped

downstream face of the Enlarged Cotter Dam (see also Fig 17).

The slump achieved in the GERCC was less than 20mm, the same

workability as that specified for all the other GERCC on the project.

Despite some initial (albeit misplaced) concerns, mainly being that slip

paving would be an impediment to overall RCC placing rates and just result in

another item of plant on the dam etc, the paver has proved to be a success. The

result is a uniformly, fully compacted 300mm (or 400mm[6]

) thick layer of GERCC inthe top step, f inished with a 1% drainage cross fall and chamfer as per design, and,

importantly, it has reduced the time involved and number of (high cost) labourers

otherwise necessary to finish the GERCC step surface by hand.

4. GALLERY CONSTRUCTION BY ROCK TRENCHER

In two recent Australian dams (Enlarged Cotter Dam and Wyaralong Dam[7])

a rock trenching machine (400HP T955 Vermeer Commanderand a 340HP 960


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Trencor respectively)with an extended boom, capable of cutting slots 460mm wide

and up to 3.9m and 3.6m deep respectively in hardened RCC, has been used in

constructing the main horizontal drainage/grouting galleries. The process of

constructing galleries by excavation is not new in the sense that some earlier

projects have excavated pre-placed non cemented sand, aggregates etc placed asa temporary fill during RCC placement, or simply trenched through the RCC while it

still has a low strength using a backhoe or excavator. Using the rock trencher has

simplified the process and allowed deeper, harder (i.e. older) RCC to be neatly

excavated and achieve design gallery height without concern.

The process involves cutting a continuous slot along each side wall and, for

wider galleries, one or more through the central zone with the narrow uncut central

portions left to be broken out and removed by excavator. Precast reinforced

concrete slabs form the roof. At Wyaralong dam the side drain along the floor was

cut by the trencher at the same time, with a high degree of accuracy being

achieved. The length of the cut gallery at the Enlarged Cotter dam was only about

40m with a cross section 3.2m wide x 3.9m high, whilst that at Wyaralong dam was

about 150m but with a far smaller section. It was found that the Enlarged Cotter

dam gallery was too short to fully achieve the anticipated benefits in terms of time

saving that would be obtained trenching a longer, smaller sectioned gallery; the

actual trench cutting operation itself took 3 shifts whilst the process of excavating,

floor trimming etc took a further 10 shifts.

Fig. 22Trialing the rock trencher at the Enlarged Cotter Dam RCC trial placement in

20MPa RCC.

At the Enlarged Cotter dam the rock trencher was trialed in the earlier

constructed full scale trial RCC placement where the RCC had already attained a

cylinder compressive strength of 20MPa, which it cut through with ease, Fig. 22.

In the dam gallery itself, the RCC strength varied from about 5-12MPa (top to

bottom) and the rock trencher achieved a neat intact RCC top edge, despite the

lower strength RCC near the top of the gallery walls. Unfortunately the top corners

were subsequently damaged by the excavator and required repair before the roof


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slabs could be placed. The final as-trenched wall surface is uniform but with a

rough texture; it allows direct inspection of the RCC, its lift joints and, in the future,

any areas of seepage should such occur, see Fig. 23.

Fig.23

The three trenched slots and two remaining upstands in the lower gallery of the

Enlarged Cotter Dam, prior to their breakout and removal of spoil by excavator and

the completed gallery.

.

5. PRE-CASTING SPILLWAY OGEE CRESTS

At the Meander Dam (50m high, 85,000m3, Tasmania Australia, 2007), the

Fig. 24

The precast concrete elements forming the crest spillway at the Meander Dam in

Tasmania, the void under them was backfilled with mass concrete[8-Fig 5].


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reduce the usual lengthy time often taken to construct these final works prior to

reservoir impoundment.

It is pleasing to see that innovation and engineering imagination is still

thriving in the RCC industry, and that new ways of meeting quality, time and costchallenges are continuing to be explored, trialed and successfully developed.

REFERENCES

[1] The International Journal on Hydropower & Dams World Atlas, 2011.

[2] Ross, Jim. Project Alliancing A Strategy for Avoiding and Overcoming

Adversity. World Project Management Week. Gold Coast, AUSTRALIA.March 2003.

[3] Forbes, Brian; Yang, Lichen; Tang, Guajin; Yang, Kangning. Jiangya

Dam China, Some Interesting Techniques Developed for high Quality

RCC Construction. Proc.of the International Symposium on Roller

Compacted Concrete Dams. Chengdu, China, April 1999.

[4] Forbes, Brian, A. RCC - New Developments and Innovations. Proc of

the 50thBrazilian Congress on Concrete-CBC2008, IstRCC Symposium.

Salvador, Brazil, September 2008.

[5] Forbes, B. A.; Iskander, M. M.; Hussein, A. I. High RCC Standards

Achieved at Jordans Tanuur Dam. International Journal on Hydropower

& Dams, Issue Three, 2001.

[6] Buchanan, Peter; Nott, Damian; Egilat, Bassam; Forbes, Brian. The Use

of 400mm RCC Lifts in The Enlarged Cotter Dam. The 6thInternational

Symposium on Roller Compacted Comcrete (RCC) Dams, Zaragoza,

Spain 2012

[7] Herweynen, R.; Stratford, C.; Watson, A. An Innovative Gallery Solutionfor an RCC Dam. International Journal on Hydropower & Dams, Issue

Four, 2011.

[8] Griggs, T.; Gibson, G. Design and Construction of the Meander Dam,

Tasmania. Proc. of the NZSOLD ANCOLD Conference. Queenstown,

New Zealand, November 2007.

[9] Herweynen, Richard; Wallis, Michael; Griggs, Tim. Spillway Design

Trends: Some Recent Australian Case Studies. ICOLD 24thCongress,

Q.94-R.4. Kyoto, Japan, June 2012.

2012 6th Int Symp on RCC Dams-Innovations of Significance and Their Development on Some Recent RCC...

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